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How does total internal reflection keep light inside an optical fibre?

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Total internal reflection keeps light inside an optical fibre by utilizing the properties of refraction at the interface between two materials with different refractive indices.

An optical fibre consists of a central core surrounded by a cladding layer that has a lower refractive index than the core. When light rays traveling through the core strike the boundary between the core and the cladding at an angle of incidence greater than the critical angle, they do not pass into the cladding. Instead, the light is completely reflected back into the core.123

Because this process repeats thousands of times along the length of the fibre, light can be "piped" or transmitted over long distances with minimal loss of intensity. This principle is fundamental to fibre optics, enabling efficient data transmission in telecommunications and the use of endoscopes in medical imaging.314